Through the years a tremendous amount of effort has been devoted to joining spaced railroad rails by some type of butt welding process. Assignee of the present application has issued patents Morlock U.S. Pat. No. 5,773,779 and Morlock U.S. Pat. No. 5,877,468, which patents are incorporated by reference herein as showing the background of the invention. These patents relate to an electric arc welding method and system for joining the ends of two spaced rails utilizing a backing between the flat bases of the rails so that the root pass or first welding layer is deposited on the backing plate spanning the gap between the rail ends as the plate is consumed. These two patents disclose in detail the total welding process above the root pass for initially joining the ends of the railroad rails and closing the gap between the rail ends. The details of such welding process do not form a part of the invention need not be repeated.
When depositing the first layer of molten metal onto the top of the backing plate between the bottom ends of the rails certain difficulties have been encountered, these difficulties affect the appearance of the weld which is to be made in the field in a very short time. The advancing welding wire used in the root pass has heretofore been moved by a robotic control in a normal weave pattern where the welding wire moves back and forth between the rails as the wire progresses along the backing plate in the bottom of the gap. This procedure causes the arc to blow through the backing plate resulting in irregular undersurface for the root pass and certain malformations in the root pass. Such blow through occurs more easily when the wire is moved too rapidly and pulls away from the weld puddle. As the robotic control moves the electrode in a robotic weave axially along the root of the gap, the arc is not pointed toward the intersection between the end of the rail and the lower backing plate. This further results in certain irregularities at this intersection. This problem is accentuated due to the fact that the backing plate is a relatively thin sheet whereas the two spaced rails are massive pieces of metal. The arc blow through of the backing plate and the inability to actually have a blow through at the intersection between the plate and rail ends has presented substantial inconsistencies in the root pass of the previously performed process. These problems affect the appearance of the root pass which is a disadvantage when attempting to use this new welding process as a replacement for prior butt welding and arc welding processes used in the field.
The present invention is a method and system for depositing molten metal from an advancing welding wire controlled by a robot to form a root pass of weld metal in the bottom of a gap between the railroad rails which gap is closed by a backing plate that is between the rails and it will be described with particular reference thereto; however, the invention has broader applications and may be used in welding the ends of railroad rails having various types of lower backing plates or in other welding operations where a root pass between two heavy metal members is deposited on the top of a relatively thin backing plate or bridging element. The present invention produces a root pass which overcomes the disadvantages heretofore experienced in producing the root pass of railroad rails by melting the backing plates as metal is deposited along the backing plate.
In accordance with an aspect of the present invention, the normal robotic weave pattern heretofore used in the gap between the rails is modified to a pendulum weave wherein the advancing welding wire controlled by a robotic mechanism is swung back and forth between the two rail ends. In this manner, the welding procedure followed by the welding wire is controlled to first swing the wire between an angle pointing toward one rail end to an angle pointing toward the other rail end. After this swinging action, the robotic mechanism moves the welding wire as it points toward the intersection between the backing plate and a rail end in a longitudinal direction. This move is a short distance, often in the range of 2-3 mm when the welding wire is about 1.6 mm. Thereafter, the swinging action is accomplished in the opposite direction bringing the welding wire to a position pointing toward the opposite intersection between the backing plate and the opposite rail end. When in the second angular position, the welding wire is again shifted longitudinally to complete a cycle of movement. The cycle is repeated to define a selected path extending along the backing plate to deposit the root pass as the backing plate is melted at least on its upper surface. The backing plate defines a barrier that produces a smooth undersurface for the resulting joint created by the root pass. In accordance with this aspect of the invention, the robotic mechanism maintains the spacing of the wire holding torch above the backing plate at a constant distance during the swinging action of the welding wire carried by the torch. By using the pendulum action and maintaining a constant height above the backing plate, the welding wire produces a consistent root welding pattern.
The selected path of the welding torch is a square weave. In the path, the torch or welding wire moves perpendicularly to the end walls of the rails. This movement is accomplished by an angular swinging action of the torch directing current to the welding wire for the welding process. Upon reaching the side wall oft he rail, the torch maintains its angle pointed toward the joint between the plates and rail end and then moves parallel to the side wall of the rail. After moving a short predetermined distance along the rail end, the robotic mechanism stops the torch carrying the welding wire. Thereafter, the torch and welding wire is pivoted or swung back toward the opposite rail. When the torch reaches the opposite rail, the angle of the torch carrying the welding wire is equal but opposite to the torch angle at the other side. Thereafter, a short forward movement is made by the torch and the cycle is repeated to create a selected welding pattern or path. By using this square wave, swinging or pendulum welding action, together with a maintenance of the height of the torch from the backing plate, advantages of the various aspects of the present invention are obtained.
In accordance with another aspect of the invention, the thin metal insert or consumable backing plate used to support the root pass as it is laid in place is held below the ends of the rail by protruding tabs and is supported on a ceramic tile. The backing plate spans the gap between the two objects being welded which, in the preferred embodiment, are the ends of railroad rails. By using a thin consumable backing plate, there is a possibility of arc blow through whereby the arc penetrates completely through the backing plate. Such arc blow through or blow through hole is not desirable as the wire moves transversely across the backing plate. The structural defect of a blow through is corrected by using the present invention. The disadvantage of this blow through is that it tends to create a cutting action as the torch swings transversely across the insert. To prevent such blow through, it is necessary to keep a puddle of metal under the arc as it is advancing between the rails. When the torch is moved too rapidly and is ahead of the weld puddle or the weld puddle flows away from the arc, the propensity for an unwanted arc blow through is substantially increased. Thus, it is advantageous to prevent the cutting action of a blow through by maintaining a wire movement speed along the selected path at a rate where a molten puddle is maintained between the arc and the plate being consumed. This can not always be done; therefore, the present invention provides an arrangement for detecting and compensating for unwanted arc blow through as the electrode or wire pivots between the ends of the rails. Blow through is detected by a monitoring system based upon a reading or sensing of the weld current. When the welding arc punches through the insert, the wire stick-out ESO from the torch increases. This causes an increase in the resistance of the welding operation. The power supply used in the welding method and system has a constant voltage control. The voltage is maintained constant by the power source. As the resistance increases during an arc blow through, the welding current has a corresponding decrease. Consequently, there is a direct correlation between the electrode stick-out and the welding current. By measuring or sensing the current, a blow through can be detected by a current decrease below a certain value. This value changes with various welding wires used in the root pass welding process. A primary aspect of the present invention provides a corrective response to an arc blow through occurring when the torch is swinging between the rails. The corrective action prevents a cutting process in the very thin backing plate. In the primary or practical implementation, the wire is moved back to the starting point of a transverse swing when a blow through is sensed. A second embodiment includes merely moving the wire back a short distance when a blow through is sensed in a transverse swing of the wire.
An automatic controller moves the torch in the selected path or pattern described in detail above. This automatic controller and the robot is responsible for movement of the torch and can adjust the torch movement based upon feedback from the welding operation. This is standard technology for controlling the movement of the electrode by the robotic mechanism for moving the torch through the selected path or pattern. When the current is sensed to be below a set value, an arc blow through is detected. When this occurs, the automatic controller in the robot mechanism moves the torch in the reverse direction back to the previous decision point or for a short distance in the selected pattern. The welding wire is melted and fills the blow through opening or hole so that the welding process can continue uninterrupted across the previously blown through hole in the backing plate. The torch returns to its movement normal pattern and is moved back across the previous blow through location. The molten metal ahead of the torch fills any hole created by the blow through. By permitting the welding process to continue a puddle of weld metal is allowed to build up under the arc. The metal fills the hole created by the blow through so that the reversal of the torch for a short time or distance or to a prior decision point fills the blow through hole at once. Thus, the blow through defect is overcome and the welding process continues. In summary, as the torch is swung back and forth, the current is monitored. If the current drops below a relatively low value, indicative of a blow through, the swinging action of the torch is reversed to fill the blow through so that the torch can resume its normal movement along the selected pass. This same action can occur as the torch is moving longitudinally between the portions of the selected pattern defined by a swinging action of the torch; however, these branches are so short and require a blow through to fill the fillet area. The invention assures at least a blow through in successive longitudinal movements on each fillet of the gap. If a blow through is detected, it is recorded. In this manner, a blow through can be forced in the next longitudinal branch on the same side of the gap when there has been no sensed blow through.
In accordance with still another aspect of the invention, the intersection or joint between the end of a rail and the backing plate is processed to provide a fillet weld as the angled torch is moved along this joint. The fillet weld is between the thick heavy end of a rail and the thin backing plate. This fillet welding aspect of the present invention can be used in various other joints where a heavy piece of metal is connected to a thin metal plate. In the fillet weld in the junction between the backing plate and the end of a rail, it is desirable to have nearly 100% penetration to produce a smooth contour on the back side of the weld. This smooth contour crucial to proper appearance of the root weld. In this fillet area of the root pass, an arc blow through is wanted to control the penetration of molten metal between the insert and the rail. If inadequate penetration occurs, the base metal at the fillet weld joint is not be completely consumed. This leads to a cold laps or notches on the backside of the weld. If there is too much penetration at this weld junction, a sharp re-entry angle may be formed from the backside of the weld. In addition, the metal overlaps the parent material of the rails forming a notch. These are disadvantages overcome by the present invention whereby a controlled arc blow through is automatically accomplished periodically along the fillet weld. The automatic control allows a weld motion controller, such as in a robot mechanism, guarantees periodic blow throughs in the fillet weld, at least in successive longitudinal branches of the weld pattern. By sensing and controlling the number and position of the periodic blow throughs an ideal weld in the fillet area of the root pass is obtained. Assurance of periodic arc blow throughs in the fillet joint is the objective of this aspect of the invention. When moving the torch along the selected path, at the intersection between the swinging action and the longitudinal movement, there is a decision point. These decision points extend along the fillet joint. When the arc reaches one of these decision points, it is determined whether there has been arc blow through as sensed by a drop in arc current. If there is a blow through, this is recorded. If there is no blow through at a first longitudinal branch in the section of the fillet joint, the torch continues to cross the gap and can come back to the next longitudinal branch on the same side of the gap. At this next longitudinal branch along the junction between the backing plates and edge of the rail a blow through must occur. In other words, there must be a blow through in accordance with this aspect of the invention at a first or second successive branch constituting the short longitudinal branches of the torch as it moves along the rail. If there is not a natural blow through at the second branch, then the voltage is decreased reducing the foot print of the arc. This causes a sharp penetration of the arc and creates a forced blow through. By having a blow through, forced or unforced, at least one time during each successive longitudinal branches along the fillet between the swinging action of the torch, a fillet joint is created that has nearly 100% penetration. This aspect of the present invention maintains a quality fillet weld in the root pass.
The present invention is performed by standard robotic control or mechanism as used to perform the welding operation in Morlock U.S. Pat. No. 5,773,779 and Morlock U.S. Pat. No. 5,877,468. In accordance with the present invention, there is provided a method of depositing metal from an advancing welding wire carried in a torch to form a root pass of weld metal in the bottom of a gap between first and second metal members, preferably railroad rails, which members have generally flat base elements joined by a metal backing plate extending between the base elements. The method comprises passing welding current between the welding wire in the torch and the metal members from the power supply to create a heat generating welding arc, moving the welding wire in a given direction along a selected path over the backing plate, and sensing an arc blow through of the backing plate by the welding arc. In one embodiment, a sensed blow through causes reversal of the given direction of the welding wire or torch for a short distance of less than about xc2xc inch when a blow through has been sensed and then resuming movement of the welding wire or torch in the given direction along the selected path. In this manner, when there is an unwanted blow through of the backing plate, the torch moves back slightly allowing molten metal to fill the blow through hole so the welding process can continue uninterrupted without cutting a kerf in the bottom backing plate. In the present implementation of the invention, there are computer recognized decision points at each change in direction of the selected path between a transverse branch and a short longitudinal branch. When a blow through is sensed, the torch or welding wire is reversed in direction and goes back to the previous decision point. The invention also senses a wanted or forced blow through in the fillet area to assure periodic blow through holes to provide excellent penetration.
In accordance with another aspect of the present invention, there are a series of decision points in the selected path as the torch moves at an angle along the fillet between the backing plate and the end of one of the rails. At each of these decision points, a determination is made whether an arc blow through, forced or unforced, has occurred. If there has been no arc blow through in two adjacent decision points in one longitudinal branch of the fillet weld, the voltage of the power supply is decreased to reduce the footprint of the arc causing a concentrated arc and thus a forced blow through. By using this aspect of the invention, in combination with the other aspect of the invention, an improved root pass is created along the upper level of the consumable backing plate and in the fillet areas.
The primary object of the present invention is the provision of a method and system for creating a root pass between the ends of two large metal members.
In accordance with still a further object of the present invention, there is provided a method and system as defined above, which method and system is easily implemented with standard robotic control mechanisms so a quality root pass can be deposited to fill the gap between the ends of the railroad rails.
Still a further object of the present invention is the provision of a method and system, as defined above, which method and system utilizes the arc current to determine when there has been an unwanted arc blow through whereby the blow through hole can be filled. Blow throughs are intentionally created in the fillet areas to produce an ideal fillet weld for the root pass.
Another object of the present invention is the provision of a method and system as defined above, which method and system senses blow through and fills the unwanted blow through holes.
Still a further object of the present invention is the provision of a method and system, as defined above, which method and system create a fillet weld between the rails and the backing plate by using an intermittent series of forced blow throughs to provide desired penetration.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawing.